Since the topic of Zurkhwood popped up, thought I'd lend my own experience with trying to play a drow druid in the past.
I saw Zurkhwood as an empty zone, figured I'd try to help make it more active when I found barbarians and rangers. Ran into two issues. The first being, my grey druid was not a Llothite. The ones who 'ran' the Grove were, and kicked out the nature walkers who weren't drow. Sure, that makes sense, but then the Grove was quiet again.
Second, the UD is full of outsider summons, pacted, undead, palemasters, animators- it was extremely challenging to navigate playing someone who hated those things but had nowhere else to go. In the end, I just rolled her due to being unable to properly play an Underdark druid. They do exist! And in written lore, they're very isolationist because of the things I just wrote above.
While I don't think it SHOULD hinge on surface relations, since both sides would be understandably wary of each other from a druid stand point, the thought process of a UD druid versus a Surfacer druid probably wouldn't be all that different. Preserve the balance, respect nature, et. cetera. The reason it does with Zurkhwood is because it's attached to Andunor and it's hard to find a neutral ground in the UD that's not tied to it in some way.
Heartwood, on the other hand, is not usually tied to another settlement. Sometimes it's tied to Myon, but not always, it kinda changes and has more opportunities. While it's more active than Zurkhwood, they definitely have their times of inactivity too. That, however, is due to druid and other naturewalker characters simply aren't that common!
If we had bigger communities of those types of toons, it'd be non-issue, but we simply don't. I only really have one idea in regards to helping UD druids seem more appealing, but from what I've seen, it's not something that the team wants to implement for valid reasons. Which is, another neutral settlement that's not Dis or Shadowvar. Drow aren't allowed in Sibayad, either. But a neutral settlement on the surface or maybe even the upper dark, with a non-alignment-locked or heavily neutral type of setting, and a Zurkhwood-replacement put there, would probably relieve that strain of druids needing to be forced to be around undead/outsiders. Though the lack of numbers would still be an issue. I can't imagine people would see a new neutral settlement and immediately get inspired to play a druid/shaman/ranger/barbarian.
Lacking that, Zurkhwood is a beautiful zone with a lot of potential. I'd hate to see it straight up deleted, but anyone able to do something special with it will be facing a major uphill battle.